Conventional snowmobile drive trains incorporate a continuously variable transmission (CVT) having a drive pulley that is operatively coupled to the engine crankshaft' and a driven pulley coupled to a driven shaft. The drive pulley transfers torque to the driven pulley via a drive belt looped around both pulleys. Typically, the driven shaft is a transverse jackshaft which drives the input member of a chain and sprocket reduction drive. The output of reduction drive is coupled to one end of an axle on which are located the drive track drive sprocket wheels.
The drive pulley includes a centrifugal actuator through which the drive ratio of the drive pulley is varied progressively as a function of the engine speed. The centrifugal actuator is connected to a movable sheave of the drive pulley. The drive pulley also includes a fixed sheave which is axially fixed. The fixed shave and the movable sheaves are rotatable together. The movable sheave is movable axially toward the fixed sheave by the action of the centrifugal actuator and away from the fixed sheave by a biasing spring. The centrifugal actuator generally consists of centrifugal weights in the form of adjusting arms. Each of the arms is connected to the movable sheave of the drive pulley by a pin, and pivots outwards about its corresponding pin. As they pivot, the arms are in contact with corresponding rollers disposed on a housing fixed relative to the fixed sheave. Surfaces of the arms in contact with the rollers are cam surfaces and the corresponding rollers are the followers of the cam surface.
When the adjusting arms pivot outwards as a result of centrifugal force, they slide against their corresponding roller and the axially movable sheave is pushed towards the fixed sheave. The axial position of the movable sheave thus depends on the angle at which the adjusting arms are pivoted, which in turn is determined by the mass and by the shape of the rolling track for the rollers on the adjusting arms, the speed of rotation of the pulley and the biasing spring characteristics. The desired interaction between the axial movement of the movable sheave and the rotational speed drive pulley is adjustable to compensate for different engines and/or operating conditions, for example when the vehicle is being operated at an altitude different from the one for which the drive pulley was originally calibrated.
There are currently several ways to adjust the interaction between the axial movement of the movable sheave and the rotational speed of the drive pulley using the adjusting arms.
One such way is described in U.S. Pat. No. 5,326,330. An adjustable eccentric member is disposed between each arm and its associated pivot point. The eccentric member is angularly adjustable to effect variations in the position of the arm relative to the pivot point. Adjusting the eccentric member alters both the position of the center of gravity of the arm with respect to the pivot point and the initial (i.e. at rest) position of the cam surface in relation to the roller.
In other actuators, such as that shown in U.S. Pat. No. 6,346,056, small weights in the form of screws are added to the arms so as to make the arms heavier and change the position of their center of gravity.
Yet another way to adjust the interaction between the axial movement of the movable sheave and the rotational speed of the drive pulley is described in U.S. Pat. No. 5,209,703. The followers have a surface with a given shape that is in contact with the arms. The followers, instead of the arms, are adjustably pivoted about their respective pivot points via one of two types of adjustments. The first type of adjustment is by pushing a pin on the followers. The pin is disposed offset of the pivot point of the followers. A translation of the pin results in change in position of the followers about their pivot point. The second type of adjustment is by using an eccentric, whereby rotating the eccentric, changes a position of the arm.
While the current drive pulleys and their adjustable mechanisms provide adequate adjustments, some of the consequences of the interaction between the axial movement of the movable sheave and the rotational speed of the drive pulley can be sometimes undesirable. FIG. 13A illustrates the force profile of an arm adjusted to several different positions. The force profile is the centrifugal force needed to lift the arm as a function of travel distance of the moveable sheave for a centrifugal actuator similar to the one disclosed in U.S. Pat. No. 5,326,330. As shown, different positions of the arms alter the force profile after zero (0) travel, which is normally the desired outcome of such an adjustment. However, the force needed to initiate the movement of the moveable sheave is also different between the different adjustment positions of the arm. A higher force needed to initiate the movement of the moveable sheave induces that the drive pulley has to be rotated at a higher speed, and as a consequence the driver has to press harder on the throttle lever. Altering the engagement speed can be in certain circumstances inconvenient to the user. The user typically becomes accustomed to a certain position of the throttle lever when initiating moving of the vehicle and normally desires for it to remain unchanged.
Therefore, there is a need for a drive pulley having a centrifugal actuator being easy to adjust. There is also a need for a centrifugal actuator that would minimally alter the engagement speed when being adjusted.